Currently available combined cycle systems of the assignee of this invention include single and multi-shaft configurations. Single shaft configurations may include one gas turbine, one steam turbine, one generator and one heat recovery steam generator (HRSG). The gas turbine and steam turbine are coupled to the single generator in a tandem arrangement on a single shaft. Multi-shaft systems, on the other hand, may have one or more gas turbine-generators and HRSG's that supply steam through a common steam header to a single steam turbine generator. In either case, steam is generated in one or more HRSG's for delivery to the condensing steam turbine.
It is well known that when a steam turbine is operating at a load below its self-sealing point, steam from an external supply (i.e., make-up steam) must be provided to the seal steam header to maintain the turbine seals until self-sealing point is reached.
At the same time, it is important that the external steam source temperature be within certain limits, depending on the inlet metal temperature of the turbine, and the load on the machine. Operation within such limits is essential to limit differential expansion in the machine, and to avoid possible thermal fatigue and other material limitations in the turbine.
Conventional techniques for supplying seal steam include:
a) Using throttle steam and attemporating (cooling) when it is too hot to meet the requirements of the steam turbine; PA1 b) Using throttle steam and operating the steam turbine for minimal time below the self-sealing point to limit the amount of time that the turbine seals are subject to high temperature steam; PA1 c) Using an intermediate pressure (IP) or low pressure (LP) header with lower steam temperatures than the throttle source. This source may be too cool, however, if the turbine is hot and operating at low load. Under this condition, the operator must limit operation at low loads to as small a time as possible; and PA1 d) Using an auxiliary boiler which is designed to provide sealing steam at an acceptable temperature for all turbine conditions. PA1 a) supplying steam from high or low pressure sections of the heat recovery steam generator to high and low pressure seals of the steam turbine via a seal steam header; and PA1 b) controlling which of the high and low pressure sections of the heat recovery steam generator supplies steam to the high and low pressure seals as a function of pressure in a low pressure header connected to the low pressure section of the heat recovery steam generator, percent turbine load and turbine inlet metal temperature.